3GPP LTE (Long Term Evolution), which is one of the next-generation cellular systems, supports a discontinuous reception (DRX) function for wireless terminals so as to reduce power consumption in each wireless terminal (NPL 1, 2). In the LTE, periods including a reception period (On-Duration), which is referred to as a DRX cycle, and a non-reception period (Opportunity for DRX) which follows the reception period, are defined. The DRX is implemented by repeating these periods.
Each wireless terminal is required to constantly receive a downlink control channel (PDCCH: Physical Downlink Control Channel) during the On-Duration period, and there is no need for the wireless terminal to receive the PDCCH during the period of Opportunity for DRX. If the wireless terminal is unsuccessful in receiving data during the On-Duration period and the data is re-transmitted after the On-Duration period, the period for receiving the PDCCH is extended.
A period in which the wireless terminal that is executing the DRX operation receives the PDCCH is referred to as an active time, and On-Duration is a minimum value of the active time. In addition, two DRX states (levels), “ShortDRX” and “LongDRX”, which have different lengths of Opportunity for DRX, can be set for each wireless terminal. In the LTE, if a wireless terminal in a ShortDRX state does not receive data for a certain period, a DRX state control for causing the wireless terminal to transit to a LongDRX state is carried out. Further, a timer (drxShortCycleTimer) is used to determine the transition from ShortDRX to LongDRX. This makes it possible to set the DRX state (level) suitable for the data reception frequency of the wireless terminal, leading to a reduction in power consumption of the wireless terminal.
Further, standardization of the LTE-Advanced is currently carried out as a cellular system in which the functions of LTE are further improved. Among the functions of the LTE-Advanced, there is a carrier aggregation (CA) in which a plurality of component carriers (CCs) are used simultaneously for a data transmission/reception with a wireless terminal, as a function for improving a peak data rate for each wireless terminal (NPL 3). The term “CC” described herein refers to a basic frequency block necessary to implement communication between a wireless base station and a wireless terminal in the LTE. In the case of performing the CA, one transport block (a data transfer unit from a MAC layer to a PHY layer) is transmitted and received using a single CC, and signal processing is carried out independently in each of the CCs. Note that if there is a need to retransmit data and HARQ is performed, the CC used for the first transmission is the same as the CC used for the retransmission.